The projection of the image of a lamp filament onto a work surface as a vane of light in order to delineate a guide line on the work surface is not new to the art. Filament image projection probably had its genesis when artists desired to determine the intersection of a plane with a curved surface. Typically, when a beginning artist attempted to draw the human figure it was difficult for the neophyte's eye initially to determine what portion of the curved body lay in any given plane. Thus, the first line projector was employed. However, there was no need for absolute accuracy, nor did it matter that the image of the filament may not have been absolutely sharp.
The use of line projectors is particularly desirable in those situations where it is either impossible, or undesirable, to strike a reference line on the work surface itself. For example, in the manufacture of automotive tires it is not only time consuming to strike an accurate, thin, highly visible reference line on the tire building drum, as well as each of the multiple layers of material wound thereon during the manufacturing process, but the dark material used in the manufacture of tires can make such a reference line quite difficult to see. In addition, the introduction of chalk, or other marking material, as a reference line can contaminate the work surface. Thus, the use of a line projector can obviate a number of problems and be quite suitable for many manufacturing operations.
As industrial applications for the use of line projectors evolved, distortion, multiple imaging and even minor misalignment could not be tolerated, and over the years a variety of rather complicated arrangements were devised whereby to adjust the orientation of the projector itself in order to attempt to effect the desired alignment of the projected image. Even though considerable strides were made in effecting a desired depth of field to the projected image--in order to accommodate non planar surfaces--as well as in eliminating double, or ghost, imaging, the means by which to adjust the orientation of the projected filament image remained quite complicated. The problem of achieving the desired orientation of the filament image was particularly complicated by the fact that even with rather rigid quality control it is quite difficult to obtain consistent orientation of the filament within the glass envelope from lamp to lamp. This problem is considerably compounded as the length of the filament is increased.
This problem can, perhaps, be most readily appreciated when one considers that an inordinate amount of time and patience was required to achieve the desired orientation of the projected filament image onto the work surface with prior art projectors. Moreover, after the desired orientation of the prior art projector had been achieved it could all be for naught when the lamp burned out and had to be replaced, because the orientation of the filament in the replacement lamp would assuredly not be the same as it had been in the previous lamp.